1. Field of the Invention
The present invention relates to magnetostrictive transducers or gauges. The magnetostrictive position transducer of the present invention includes an improved signal reception subsystem, which improves noise discrimination performance.
2. Description of the Prior Art
Magnetostrictive position sensor devices are widely used in the measurement and control industry. They find use in machine tools and robotics as well as other applications which call for accurate position indication.
The term magnetostriction usually refers to the dimensional change of a ferromagnetic body that occurs during magnetization. In general, magnetostrictive position sensors incorporate a ferromagnetic delay line, or "waveguide". A pulse generator supplies a current pulse to the delay line which generates a magnetic field which surrounds the delay line. A remote and movable, position indicating magnet, is placed at a position along the delay line. The magnetic field of the position indicating magnet interacts with and disturbs the magnetic field generated by the current pulse.
The interaction between the permanent magnetic field of the position indicating magnet and the magnetic field induced by the current pulse causes a strain or mechanical reaction within the delay line. This induced reaction force within the ferromagnetic delay line is propagated along the length of the delay line as an acoustic torsional wave, in accordance with the Wiedemann effect, which is known to those skilled in the art.
Since the speed of propagation in the delay line is fixed, one can determine the location of the acoustic wave source by measuring the time required to receive the delayed acoustic pulse. In operation, the time interval between the excitation pulse, initiated by the sampling clock, and the reception of the delayed acoustic pulse returned from the position indicating magnet magnet, indicates the location of the position indicating magnet.
A transducer device, called a mode convertor, is typically attached to one end of the waveguide. The mode convertor responds to the passage of the torsional acoustic wave and converts the torsional acoustic wave into a representative electrical signal. Typically, the mode convertor device operates on the Villari or inverse magnetostrictive effect, where the application of strain alters the magnetic properties of the convertor.
The time delay period from the excitation of the waveguide to the delayed reception of the corresponding acoustic wave at the mode converter can be measured in a variety of ways. Also, the time delay can be converted into a position indicating signal in a variety of ways. Traditionally, position output information is available in a variety of formats.
For example, U.S. Pat. No. 3,898,555 to J. Tellerman uses a fixed frequency oscillator to excite the delay line. The returned acoustic signal, in conjunction with the fixed frequency oscillator, develops a "digital" signal which is "pulse width modulated" by the position of the magnet along the delay line. An integrator converts the pulse width modulated waveform to a DC voltage level which is delivered as the transducer's "analog" output signal.
U.S. Pat. No. 4,721,902 to J. Tellerman et al. teaches, inter alia, a method to convert the "pulse width modulated position signal" into a digital value. The patent teaches the use of a conversion counter to collect "counts" from a conversion oscillator during the "on" time of the pulse width modulated signal.
This patent also teaches a method to enhance the detection of the delayed acoustic signal through the use of a time domain filter which sets the duration of an inhibit timer based upon the historical output of the transducer. This time domain filtering technique eliminates the contribution of noise to the output signal; however, it effectively limits the maximum rate at which the position indicating magnet can move along the gauge.
Successful use of the magnetostrictive measurement technique requires the reliable detection of the delayed acoustic pulse. These acoustic pulses are attenuated during the course of transmission in the waveguide. In general, the amplitude of the acoustic pulses are the greatest when position indicating magnet is closest to the mode converter; the acoustic pulses are faintest when the magnet is remote from the mode converter. For example, signal attenuation approaches 60% as the stroke length reaches 60 feet. In this instance, noise originating near the mode converter end of the gauge can approach 30% of the signal. This factor makes the construction of long gauges problematical.
Noise sources affect the operation of the gauge as well. External or environmental factors which can alter the stability of the signal. For example, external electric and magnetic fields within the operating environment of the device may decrease the signal-to-noise ratio and make discrimination of the return signal more difficult. Internal noise sources such as magnetic abnormalities, discontinuities and foreign particles in the waveguide material contribute background noise to the waveguide.
These considerations show that the detectability of the signal is marginal on long gauges, where prior art detection circuits are used. As a consequence the maximum length of a magnetostrictive gauge was limited by the detectabilty, in the presence of noise, of the delayed acoustic pulse.